Since 1928 and 1929 when Edward H. Angle first publicly described his invention of the edgewise appliance, the single width bracket he invented has been modified to correct tooth rotations in the horizontal plane. Since the 1990s the width of the brackets typically used by orthodontists have decreased over time. Two other dimensions of Angle's original rectangular edgewise slot, height of 22 mils and depth of 28 mils (one thousandths of an inch) are used by many orthodontists currently. A smaller 18 mil by 25 mil slot was invented and is used by a large number of orthodontists. In some cases, patient comfort and cosmetic appeal have motivated the decrease in width and size of these various components of the edgewise appliance.
While these smaller components have been used extensively in the past, they do have some drawbacks. First of all, while not wishing to be bound by theory, the applicant opines that when retracting upper canines in a typodont, using a nickel-titanium spring delivering initially 150 grams of force, the canine with a twin edgewise bracket (22 by 28 mil slot) and bracket width of 4.6 mm moved in a bodily manner along the 20 mil arch wire. In contrast, the canine with a twin edgewise bracket (22 by 28 mil slot) and bracket width of 2.2 mm experienced with the same spring force controlled tipping. Explanation: In the edgewise bracket the tooth is translated by a series of opposite rotations about the center of resistance (CR) in the root and about the center of the bracket. As a bracket is reduced in width and size, the lever arms within the bracket are also reduced in size which requires greater forces to operate them, hence the retraction force rotating the tooth about its CR overpowered the ability of the bracket and arch wire to create a couple to rotate the tooth about the center of the bracket. A wider bracket can more effectively translate a tooth along an arch wire than a narrower bracket. For both wide and narrow brackets, use of heavy forces probably increase the potential for undesirable consequences such as high amounts of root resorption. Narrow brackets are useful in the initial stages of tooth alignment because they have larger inter-bracket distances that allow flexible arch wires to properly align the teeth.
Consequently, there may exist a need for improved methods and systems for translating of teeth along an arch wire while minimizing rotation of teeth in an edgewise appliance.